Conventionally, in an LED made of a nitride semiconductor, GaN is mainly used. In this case, from the viewpoint of lattice matching, a GaN film having few defects is formed on a sapphire substrate by epitaxial growth, so as to form an LED element made of a nitride semiconductor. Here, the sapphire substrate is an insulating material, so that, for the purpose of supplying electric power to a GaN-based LED, the n-layer is exposed by cutting a part of the p-layer to form an electrode for power supply on each of the p-layer and the n-layer. The LED having a structure such that the electrodes for power supply are arranged in the same direction is referred to as a lateral-type structure, and such a technique is disclosed, for example, in the following Patent Document 1.
On the other hand, for the purpose of improving the light emission efficiency and achieving more efficient light extraction of the LED element, development of an LED having the so-called longitudinal-type structure in which the p-layer and the n-layer are arranged on the front and back surfaces to provide power supply is advanced. In producing an LED having this longitudinal-type structure, the n-layer and the p-layer are arranged on the sapphire substrate sequentially from below and, after a support substrate made of silicon (Si) or copper tungsten (CuW) is bonded to the p-layer side, the sapphire substrate is removed. In this case, the element surface will be the n-layer side, and the voltage supply is carried out by disposing a power supply terminal on this n-layer and connecting a wire, which is a power supply line, to this power supply terminal.
In the longitudinal-type structure, when a voltage is applied between an electrode on the p-layer side (hereafter referred to as “p-side electrode”) and a bonding electrode formed on the n-layer (hereafter referred to as “n-side electrode”), a current flows from the p-side electrode to the n-side electrode via the LED layer including the light-emitting layer. By the flow of the current in the light-emitting layer, a region of the light-emitting layer emits light.
Here, the p-side electrode and the n-side electrode are arranged in a positional relationship of opposing each other in a vertical direction. For this reason, when a voltage is applied between the two electrodes, a current path in the vertical direction of travelling almost by the shortest distance from the p-side electrode towards the n-side electrode is formed. In this case, almost all of the current flows in a part of the light-emitting layer located vertically under the n-side electrode, and little current flows in other parts of the light-emitting layer, thereby causing a problem in that the light-emitting region is restricted to lower the light emission efficiency.
In view of such a problem, the following Patent Document 2 discloses a construction in which the n-side electrodes are disposed in a network configuration. By disposing the n-side electrodes in a network configuration, the current flowing in the light-emitting layer can be widened in a horizontal direction while ensuring the light extraction region.
Also, when the resistance value between the LED layer and the n-side electrode is high, another problem is caused such that the voltage needed for allowing the current needed for light emission to flow is high. For this reason, in order to extract light of a high light quantity with a low operating voltage, it is important that the resistance value between the LED layer and the n-side electrode is reduced as much as possible. In view of such a problem, the following Patent Document 3 discloses an LED element in which the n-layer in the LED layer is formed by sequential lamination of a high-concentration layer and a low-concentration layer.